1. Field of Invention
This invention relates to a sensor which detects approaching metal without actually contacting the metal. More specifically, this invention relates to an oscillation circuit including an LC resonant circuit that serves as a detection circuit.
2. Related Art
The sensor of this invention can be installed, for example, in a pachinko machine. When a pachinko ball goes through a payoff hole, the sensor detects the passage and, accordingly, the machine pays off a certain amount of balls to an operator. A conventional sensor is an induction type sensor. When metal approaches an induction coil of a conventional sensor, an eddy current is induced in the metal and resistance is raised in the coil. Consequently, the metal is detected.
Other sensors used for detecting the existence of pachinko balls are a contact type and an optical type. The contact type consists of a movable lever which is movably displaced by the passage of the pachinko ball. When the lever is fully displaced it contacts a contact and the machine is signaled that a ball has passed. The lever is biased in an off, or closed, position and only makes contact, assuming an on state, when a ball passes. However, such contact type sensors have drawbacks in that chattering occurs, potentially causing miscount of the actual number of balls passing through the opening. The optical sensor detects the pachinko ball when it passes through a beam emitted from a light emitting device. When the ball passes through the beam, a phototransistor does not receive the emitted light and detection is indicated. However, optical sensors have drawbacks in that "noise" interferes with the count.
Thus, induction type sensors are usually implemented in pachinko machines, avoiding the drawbacks of contact and optical sensors. Specifically, induction sensors are subjected to a reduced effect from outside "noise" and have a prolonged life because they can detect the ball without actually contacting it.
FIG. 9 illustrates the general appearance of an induction type pachinko ball sensor 11. When a pachinko ball 14 goes through a detection hole 13 in casing 12, which is made from synthetic resin, the energy of a coil L incorporated in the casing 12 and surrounding the hole 13 is changed and a detection circuit 19, also inside the casing 12, detects the passage of the ball 14.
FIGS. 10 and 11 show the principle of a sensor. When a metal piece 15 is far away (at a distance 11) from the coil L, an oscillation circuit's 16 output (V out) oscillates. However, when the metal 15 approaches (to a distance 12), the oscillation circuit's 16 output (V out) no longer oscillates because the metal 15 is subjected to magnetic flux from the coil L. Consequently, the metal 15 produces an eddy current due to electron conduction. The electron conduction increases resistance in the oscillation coil L and oscillation stops, as shown in FIG. 11 (2).
FIG. 12 shows a Hartly type oscillation circuit used as the oscillation circuit 16 in FIG. 11. This oscillation circuit 16 comprises a transistor TR11 for amplification, an LC resonant circuit 17, and resistor R12. The operation point of transistor TR11 is determined by arranging a constant current source 21 and bias voltage Vs. Transistors TR12 and TR13 comprise a mirror circuit. The collector circuit of TR13, which has the same bias as the collector current of TR11, is fed back to the LC resonant circuit 17. Additionally, power source voltage is Vcc, output of the oscillation circuit is Vout, and G is the ground.
As shown in FIG. 13, a metal body 15 is approaching or going away from the sensor containing the above-mentioned oscillation circuit 16. The output signal Vout, shown in FIG. 14, changes with respect to metal's proximity to the sensor. FIG. 14 (1) shows the oscillation coil output while FIG. 14 (2) represents binarization of the coil's output. The Vout is binarized according to a predetermined threshold level; binary signal S corresponds to the periods of reduced oscillation from the coil that occur when metal is in close proximity to the sensor, i.e. when the pachinko ball 14 goes through the detection hole 13.
In addition to the Hartly type oscillation circuit, a Corbitt type oscillation circuit, as shown in FIG. 15, may be used to detect pachinko balls. In a Corbitt type oscillation circuit, a pair of capacitors C1 and C2 and oscillation coil L comprise LC resonant circuit 20. Additionally, LC resonant circuit 20 and a plurality of resistors R13, R14, R15 are electronically connected to a pair of transistors T14, T15.